338 THE PALEONTOLOGY OF MINNESOTA. 



f LLIngula. 



Nepionic, Hyatt.* The smooth shell stage succeeding the protegulum. 



Nealogic, Hyatt. Youthfulness, or the stage in which specific characters begin to 

 develope. 



Ephebolic, Hyatt. The mature shell. 



Geratologic, Hyatt, Old age. It is indicated in many species of brachiopods by 

 extreme thickness of the valves, obesity, or by numerous, crowded 

 growth lines near the anterior margin, a condition which sometimes 

 produces truncation and absence of striae at the margin. 



Class BRACHIOPODA, (Cuvier) Dumeril. 



Subclass LYOPOMATA, Owen. 



Order ATKEMATA, Beecher. 



Family LINGULIOE, Gray. 



Genus L1NGULA, Bruguiere. 



1789. Lingula, BRUGIERE. Histoire naturelle des Vers Testacs. 

 1892. Lingula, HALL. Paleontology of New York, vol. viii, pt. i, p. 2. 



Description: "Shells subequivalve, equilateral; elongate-ovate, subquadrate 

 or subtriangular in outline; broad over the pallial region, cardinal slopes more or 

 less conspicuous; slightly gaping at both extremities. Brachial or dorsal valve 

 somewhat the shorter, and with a slightly thickened hinge-line. Surface of the 

 shell smooth, or concentrically and radiately striated. Animal attached by a long, 

 muscular pedicle protruding from between the beaks of the two valves. 



" Muscular impressions numerous, but usually indistinct. In the recent species 

 they are twelve in number upon each valve, and are somewhat unsymmetrical in 

 their arrangement. They may be designated as follows : The umbonal impressions, 

 produced by a single muscular band passing directly across the cavity of the shell 

 near the beaks, and by their contraction opening the valves; the lateral impressions, 

 which are produced by three pairs of muscles, the (interiors passing from near the 

 lateral boundaries of the visceral area on the pedicle [ventral] valve, forward to the 

 anterior extremity of this tract on the brachial [dorsal] valve; the middles passing 

 in just the opposite direction, from the anterior region of the pedicle-valve to the 

 lateral region of the brachial; the externals passing from the ante-lateral region of 

 the pedicle valve to the post-lateral region of the brachial valve; these muscles 

 serving to move the valves forward and backward. The central impressions are 



* Values in classification of the stages of growth and decline, with propositions for a new nomenclature," by Alpheus 

 Hyatt; Am, Nat., vol. xxli, p. 872, 1888. Also " Genesis of the Arietldse." Mem. Mus. Comp. Zool., vol. xvi, no. 3, 1889. 



